Ca facilitates secondary metabolism
Apart from the aforementioned primary metabolism process, three secondary metabolism related proteins were activated by exogenous Ca (Table 1), whose abundance increased significantly at high Ca level compared with low Ca or medium Ca treatment. The involved metabolites included methylglyoxal, vitamin, isoprenoid and turpentine.
Lactoylglutathione lyase (spot 6) participates in the detoxification of methylglyoxal, and a higher transcription of lactoylglutathione lyase has been reported in aluminum treated tomato root.49 Methylglyoxal is a by-product of a number of metabolic pathways, especially glycolysis.50 Plant usually maintains low level methylglyoxal under normal growth conditions while abiotic stresses induce dramatically increase of methylglyoxal, which functions as a toxic molecule and inhibits a variety of biological processes in plant, such as seed germination, photosynthesis and root growth.51 High Ca treatment induced higher expression of lactoylglutathione lyase possibly accelerated the detoxification and remove of excessive methylglyoxal, which is an unavoidable by-product of Ca improved various metabolism processes, such as glycolysis, in P. massoniana seedlings.
Both sopentenyl diphosphate isomerase (IDI)-I and IDI-II participate in the rate-limiting step for the biosynthesis of terpenoid compounds, which are important components of turpentine in pinaceae plant.52 In the present study, Ca deficiency down-regulated the expression of IDI- II like (spot 12) while high Ca supply recovered its expression to normal level (Table 1). Adequate exogenous Ca supply possibly promoted the biosynthesis and accumulation of turpentine in P. massoniana through restoring IDI mediated terpenoid biosynthesis, which is of production guidance significance in P. massoniana forest industry for turpentine yield.
Thiamine thiazole synthase involves in the biosynthesis of thiazole, which is the precursor of thiamine (vitamin B1).53 The significant up-regulated thiamine thiazole synthase 2 (spot 63) implied high concentration of exogenous Ca benefited various glycometabolism and energy pathways may be related to the enhanced biosynthesis of thiamine, as thiamine is an essential cofactor for the enzymes activating a plenty of carbohydrate metabolism pathways.53
The mechanisms underlying high Ca treatment resulted strengthened secondary metabolism could be attributed to the following potential factors. Primarily, abundant exogenous Ca promoted the processes of photosynthesis and diverse primary metabolism pathways, which laid a sound material and energy foundation for the anabolism of secondary metabolites. Besides, the expression of some secondary metabolism related proteins is a response to high Ca motivated primary metabolism, with the purpose of removing excessive by-products of primary metabolism or synthesizing cofactors for primary metabolism.
In addition, based on the difference in Ca requirement for plants and Ca concentrations in natural habitats, plants can be classified into calcifuge plant and calcicole plant.16 It can be deduced that P. massoniana is a calcicole plant to some extent considering the improved various material metabolism pathways, facilitated photosynthesis, increased biomass and better growth at high Ca level.